displaying recent subcategories's topics categories

I am going to use a lot of categories and subcategory in the BBPress forums, how can I make displaying the most recent 15 subcategories’s topics in my categories? For example, the main forum’s page displays [bbp-forum-index] and [bbp-topic-index] , if I am goint to category’s page, this page displays only subcategories, and i want to display resent topics under list of subcategories! Sorry for my English